1. Field of the Invention
The present invention relates generally to microminiature electronic elements and particularly to an improved design and method of manufacturing microminiature electronic components including toroidal transformers and inductive reactors (i.e., xe2x80x9cchoke coilsxe2x80x9d).
2. Description of Related Technology
For many years, electronic circuit boards have been fabricated by interconnecting a plurality of electronic components, both active and passive, on a planar printed circuit board. Typically, this printed circuit board has comprised an epoxy/fiberglass laminate substrate clad with a sheet of copper, which has been etched to delineate the conduct paths. Holes were drilled through terminal portions of the conductive paths for receiving electronic component leads, which were subsequently soldered thereto.
More recently, so-called surface mount technology has evolved to permit more efficient automatic mass production of circuit boards with higher component densities. With this approach, certain packaged components are automatically placed at pre-selected locations on top of a printed circuit board so that their leads are registered with, and lie on top of, corresponding solder paths. The printed circuit board is then processed by exposure to infrared, convection oven or vapor phase soldering techniques to re-flow the solder and, thereby, establish a permanent electrical connection between the leads and their corresponding conductive paths on the printed circuit board.
The increasing miniaturization of electrical and electronic elements and the high density mounting of such elements has created increasing problems with electrical isolation and mechanical interconnection. As circuit board real estate becomes increasingly more valuable, more and more components are put into increasingly smaller spaces, thereby generally increasing the heat generation per square millimeter of circuit board, as well as the likelihood of electrical and electromagnetic interference (EMI) between components in such close proximity. Such factors strongly militate in favor of components that utilize the absolute minimum footprint, and have acceptable heat and EMI signatures in addition to the desired electrical performance.
One very commonly used component is the transformer. As is well known in the art, transformers are electrical components that are used to transfer energy from one alternating current (AC) circuit to another by magnetic coupling. Generally, transformers are formed by winding one or more wires around a ferrous core. One wire acts as a primary winding and conductively couples energy to and from a first circuit. Another wire, also wound around the core so as to be magnetically coupled with the first wire, acts as a secondary winding and conductively couples energy to and from a second circuit. AC energy applied to the primary windings causes AC energy in the secondary windings and vice versa. A transformer may be used to transform between voltage magnitudes or current magnitudes, to create a phase shift, and to transform between impedance levels.
Another purpose for which microelectronic transformers are commonly used is to provide physical isolation between two circuits. For example, a transformer may be used to provide isolation between a telephone signal line and the Central Office (CO), and in the public switched telephone network and consumer equipment such as modems, computers and telephones, or between a local area network (LAN) and a personal computer. Often, the transformer must be able to withstand large voltage spikes which may occur due to lightning strikes, malfunctioning equipment, and other real-world conditions without causing a risk of electrical shock, electrical fire or other hazardous conditions.
In furtherance of these ends, the electrical performance of the transformer must be carefully considered. One means by which the electrical performance of transformers is gauged is the Dielectric Withstanding Voltage (DWV) or hi-pot test. A hi-pot test involves the application of AC or DC signals to the transformer to determine whether the breakdown of the core dielectric or other destructive failures occur at some chosen voltage level. A hi-pot test can also be used to demonstrate that insulation can withstand a given over-voltage condition (such as the aforementioned voltage spikes) and to detect weak spots in the insulation that could later result in in-service failures.
The International Electro-Technical Commission is an international standards body that develops the standards by which isolation transformers are categorized according to level of safety. Underwriter""s Laboratories Standard 1950 (UL-1950) is the corresponding harmonized national adaptation for the United States. It specifies a minimum standard for dielectric breakdown between the primary and secondary windings of a transformer. Under UL-1950, insulation systems used in transformers are classified as Operational, Basic, Supplementary, or Reinforced. The most common classification for transformers used in telecommunications application is Supplementary.
In order to meet a standard such as UL-1950, it is critical that the primary and secondary windings are electrically isolated and/or physically separated from one another while remaining magnetically coupled to one another through the transformer core. The standard provides for (or allows) the use of: (1) required minimum spacing distances, (2) minimum thickness of solid insulating material, or (3) a minimum number of layers of a thin film of insulation for compliance. When the use of layers of a thin film of insulation is the means selected to provide electrical isolation between windings in the transformer, the standard states that a minimum of two layers must be used. Each of the layers must individually pass the DWV requirement. Three layers may also be used, in which case the DWV requirement must be met by testing combinations of two layers at a time. An option provided under the standard is to apply the thin films directly to a conductor as in the case of a wire having two or three extrusions of film material deposited directly over the copper conductor.
Magnet wire is commonly used to wind transformers and inductive devices (such as inductors or choke coils). Magnet wire is made of copper or other conductive material coated by a thin polymer insulating film or a combination of polymer films such as polyurethane, polyester, polyamide, and the like. The thickness and the composition of the film coating determine the dielectric strength capability of the wire. Magnet wire in the range of 31 to 42 AWG is most commonly used in microelectronic transformer applications, although other sizes may be used in certain applications.
Note that where Supplementary or Reinforced insulation is required by the cognizant safety agencies for specific applications, such as in the case of the aforementioned UL standard, the enamel insulation used on magnet wire is generally not sufficient. In these cases, the transformers need to be built such that additional insulation between the windings is provided. This is often achieved by adding insulating tape between the windings and additional tape in the margins of the winding form to provide spacing to ensure that the required minimum distance between the primary and secondary windings is maintained. While useful in certain types of transformers, such xe2x80x9cmarginxe2x80x9d tape is not well adapted to very small transformers, and toroidal cores in particular.
Hence, under the prior art, the designer is left with the choice of using margin tape and layers of thin insulation or individually insulated wires in order to meet the dielectric requirements set forth in the applicable standards. One major disability with the use of individually insulated wires in transformer applications is space. Specifically, since each conductor is insulated with its own layers of insulation (typically on the order of a few mils thickness), it can be readily appreciated that the space required by many layers of such conductors wound atop each other is very much greater than that required by the same size (e.g., AWG) conductors without the insulation. Hence, any transformer which uses individually insulated conductors such as those described above would necessarily be much larger in size that a comparable transformer without insulation, if the latter could be made to work and still meet its electrical performance and safety agency requirements.
FIG. 1a illustrates one prior art microelectronic transformer arrangement commonly used, often referred to as a xe2x80x9cshapedxe2x80x9d core. The core 102 of the device 100 of FIG. 1a is formed from two half-pieces 104, 106, each having a truncated semi-circular channel 108 formed therein and a center post element 110, each also being formed from a magnetically permeable material such as a ferrous compound. As shown in FIG. 1, each of the half-pieces 104, 106 are mated to form an effectively continuous magnetically permeable xe2x80x9cshellxe2x80x9d around the windings 112a, 112b, the latter which are wound around a spool-shaped bobbin 109 which is received on the center post element 110. When completely assembled, the device 100 is mounted on top of a terminal array 114 generally with the windings 112a, 112b (i.e., the truncated portions 116 of the half-pieces 104, 106) being adjacent to the terminal array 114, which is subsequently mated to the printed circuit board (PCB) when the device 100 is surface mounted as shown in FIG. 1b. Note that the truncated portions are present, inter alia, to allow termination of the windings 112 outside of the device 100. Margin tape 119 is applied atop the outer portions of the outer winding 112b for additional electrical separation.
FIG. 1c illustrates a cross-section of the device 100 after assembly, and accordingly some of the disabilities associated with this design. As shown in FIG. 1c, the magnetic coupling between the permeable half-pieces 104, 106 and the windings is non-optimized because of the presence of the truncated portion 116 consisting of insulating tape. In addition, the design of FIGS. 1a-1c is not optimized in terms of volume and footprint. A significant amount of volume is devoted not only to the half-pieces 104, 106, semi-circular channel 108, and bobbin 109, but also to the windings themselves. As previously described, it is common to use either individually insulated conductors and/or margin tape in order to provide the desired degree of insulation between the windings 112a, 112b of the device 100, both of which require substantial additional space.
In terms of footprint, even when the device is oriented with respect to the terminal array 114 and PCB 120 as shown in FIGS. 1b and 1c (which arguably requires the smallest footprint on the PCB when compared to other possible orientations of the half-pieces 104, 106), the size of the footprint 122 is still comparatively large, owing in substantial part to the use of individually insulated conductors and/or margin tape.
Other disabilities associated with the transformer arrangement of FIGS. 1a-1c include the necessity to accurately align the two halves 104, 106 of the core during manufacturing, as well as the requirement that the mating surfaces of the two halves be very smooth and planar. As is well known, the alignment of the two magnetically permeable halves of the shaped core will affect the magnetic (and therefore electrical) performance of the device; imperfect alignment or matching of the halves causes spatial variations in the flux density, and therefore also in the energy coupled between the windings. Similarly, if the mating surfaces of the halves are not smooth and planar (i.e., flat), variations in magnetic coupling occur as well. Such variations can be significant in magnitude, and can result in substantial variations in the electrical performance of one device as compared to another manufactured using the same process. Ideally, all transformers manufactured using the same components and processes would have identical electrical performance; hence, the foregoing inherent variations in the shaped core transformer make it a less-than-perfect design from a performance standpoint. When coupled with the aforementioned spatial restrictions, and the additional labor required to make use of individually insulated conductor and/or margin tape, the shaped core design becomes even less desirable.
Another well-known configuration for a microelectronic transformer comprises a toroidal ferrite core. A toroidal transformer can readily be adapted to provide any one of the transformer functions listed above. One significant drawback to the use of toroidal cores, however, is the inability to use the device in conjunction with individually insulated conductors (e.g., additional insulation such as a Teflon(copyright) coating disposed over or in place of the normal polyurethane or similar coating on the conductors) or margin tape. While a microelectronic toroidal core may be successfully wound with primary and secondary windings comprising fine gauge magnet wire, the use of more heavily insulated windings is precluded based on the limited size of the device. Furthermore, it is exceedingly difficult to utilize margin tape on a toroid, since it significantly limits the winding area (i.e., xe2x80x9cwindowxe2x80x9d), and cannot be placed on the core mechanically as on a bobbin, but rather must be manually placed. Manual placement such as this greatly increases the cost of manufacturing each device. In addition, placement of the windings on the toroid would have to be such that the required electrical performance and separation parameters could not be satisfied. Hence, prior art toroid core transformers that are required to meet the stringent dielectric performance requirements previously discussed are practically limited to a certain minimum size, which is often much too large for the desired application.
It should be noted that an additional consideration in choosing between the aforementioned prior art xe2x80x9cshapedxe2x80x9d core and toroidal core configurations relates to the use of an air gap within the transformer for control of core saturation. Designers have heretofore been generally forced in the direction of using a shaped core as opposed to a toroidal core in such applications, since the use of an air gap in the toroid core has presented difficulties not existing in the shaped core. Specifically, the mechanical reliability of gapped toroids has been questionable at best, and the cost of producing these components significantly higher than a shaped core transformer of equivalent capability. Furthermore, only a very limited number (i.e., one) of vendors currently produce such a component. These practical barriers to the use of a toroid core transformer with an air gap have accordingly restricted the options open to the designer when designing a transformer for a specific application, a potentially severe disability in cases such as where the reduced size or other desirable features of the toroid core are required.
Based on the foregoing, it would be most desirable to provide an improved microelectronic component and method of manufacturing the same. Such an improved device would provide a high dielectric strength between individual windings of the device (such as the primary and secondary windings of the aforementioned toroidal core transformer), while occupying a minimum volume. Additionally, such improved device would have a minimal footprint (or alternatively, larger footprint and lower vertical height from the substrate), and could be manufactured easily and cost-efficiently, with little or no variation in electrical performance from device to device. Such device would also readily accommodate an air gap if desired by the designer, without other adverse effects.
The present invention satisfies the aforementioned needs by providing an improved microelectronic device, and method of manufacturing the same.
In a first aspect of the invention, an improved microelectronic toroidal element for use in, inter alia, surface mount applications and microelectronic connectors is disclosed. In one exemplary embodiment, the toroidal element comprises a transformer having a toroidal core fashioned from magnetically permeable material; a first winding (e.g., primary) wound around the toroid in a layered fashion; a layer or a plurality of layers of polymeric insulating material (e.g., Parylene) formed over the top of the first winding; at least one second winding (i.e., secondary) wound around the toroid and over the top of the insulating material. The application of the insulating material is controlled such that the required dielectric properties are obtained over the length of the windings including the free ends that terminate external to the element. A vacuum deposition process is advantageously used for the application of the Parylene thereby providing the maximum degree of uniformity of material thickness, which in turn allows for the smallest possible physical profile of the device. One or more gaps are also optionally provided in the toroidal core so as to meet electrical and magnetic parameters such as energy storage and minimal changes over temperature.
In a second aspect of the invention, an improved microelectronic package incorporating the aforementioned toroidal element is disclosed. In one embodiment, the package comprises a toroidal core transformer having a gap, first winding, Parylene insulation layer(s), and second winding as described above, the toroid being mounted on terminal array in a vertical orientation (i.e., such that the plane of the toroid is normal to the plane of the terminal array and the substrate to which the latter may be affixed) with respect thereto. The free ends of the first and second windings are conductively joined with the conductive terminals of the terminal array, thereby forming a conduction path through each of the transformer windings to and from the traces or vias of the substrate. The toroid is advantageously held in place by the tension of the free ends of the windings being joined to the terminals of the array, thereby obviating the need for a separate retention mechanism. The package is also optionally encapsulated with a polymer encapsulant for enhanced mechanical strength and environmental isolation. In a second embodiment, one or more toroid elements are disposed within a mounting base (such as an xe2x80x9cinterlockxe2x80x9d base), the latter having a plurality of preformed lead channels in which are received respective electrical leads used for mounting the package to the substrate. The toroid windings are coated up to the point of entering the lead channels, thereby assuring adequate electrical separation between the toroid and the winding egress. The mounting base, including toroid and windings, are also optionally encapsulated.
In a third aspect of the invention, an improved circuit board assembly incorporating the aforementioned microelectronic package is disclosed. In one exemplary embodiment, the assembly comprises a substrate having a plurality of conductive traces disposed thereon with the microelectronic assembly bonded thereto such that the leads or terminals of the package are in contact with the traces, thereby forming a conductive pathway from the traces through the toroid windings of the package.
In a fourth aspect of the invention, an improved microelectronic connector assembly incorporating the aforementioned toroid element is disclosed. In one embodiment, the connector comprises an RJ-type connector (e.g., RJ-11 or RJ-45) having a body and a receptacle formed therein, the receptacle having a plurality of electrical contacts for mating with the contacts of a modular plug received within the receptacle; a cavity disposed within the body; and at least one toroid element having a plurality of windings of the type previously described disposed with the cavity. One set of windings of the toroid is coupled to the terminals of the aforementioned electrical contacts, thereby forming a conductive pathway from the contacts of the modular plug through the contacts and terminals of the connector and through the windings of the toroid element. A set of leads connecting the second set of toroid windings to an external device (such as a PCB) are also provided. The cavity of the connector is optionally filled with an epoxy or other encapsulant if desired.
In a fifth aspect of the invention, an improved method of manufacturing the toroid core element of the present invention is disclosed. The method generally comprises the steps of providing a toroidal transformer core; forming a gap within the core; winding the toroidal transformer core with a first set of windings; depositing on the first set of windings at least one layer of an insulating coating; winding the core with a second set of windings; and terminating the first and second sets of windings to a terminal array. In one embodiment, the insulating coating is Parylene, a thermoplastic polymer, which is deposited on the first set of windings using a vacuum deposition process. The toroid elements with first winding are hung from a lateral support member within the vacuum deposition chamber such the desired length of leads is exposed to the deposition process. A layer of insulating material is also optionally deposited over the core before the first set of windings is applied in order to mitigate chafing or abrasion of the conductors during the winding process. After the second set of windings is applied over the toroid, the device is terminated and optionally encapsulated with an epoxy or other encapsulant.